Re: [PATCH RFC 03/39] KVM: x86/xen: register shared_info page

[David Woodhouse <dwmw2@infradead.org>]

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On Tue, 2020-12-01 at 16:40 -0800, Ankur Arora wrote:
> On 2020-12-01 5:07 a.m., David Woodhouse wrote:
> > On Wed, 2019-02-20 at 20:15 +0000, Joao Martins wrote:
> > > +static int kvm_xen_shared_info_init(struct kvm *kvm, gfn_t gfn)
> > > +{
> > > +       struct shared_info *shared_info;
> > > +       struct page *page;
> > > +
> > > +       page = gfn_to_page(kvm, gfn);
> > > +       if (is_error_page(page))
> > > +               return -EINVAL;
> > > +
> > > +       kvm->arch.xen.shinfo_addr = gfn;
> > > +
> > > +       shared_info = page_to_virt(page);
> > > +       memset(shared_info, 0, sizeof(struct shared_info));
> > > +       kvm->arch.xen.shinfo = shared_info;
> > > +       return 0;
> > > +}
> > > +
> > 
> > Hm.
> > 
> > How come we get to pin the page and directly dereference it every time,
> > while kvm_setup_pvclock_page() has to use kvm_write_guest_cached()
> > instead?
> 
> So looking at my WIP trees from the time, this is something that
> we went back and forth on as well with using just a pinned page or a
> persistent kvm_vcpu_map().

OK, thanks.

> I remember distinguishing shared_info/vcpu_info from kvm_setup_pvclock_page()
> as shared_info is created early and is not expected to change during the
> lifetime of the guest which didn't seem true for MSR_KVM_SYSTEM_TIME (or
> MSR_KVM_STEAL_TIME) so that would either need to do a kvm_vcpu_map()
> kvm_vcpu_unmap() dance or do some kind of synchronization.
> 
> That said, I don't think this code explicitly disallows any updates
> to shared_info.

It needs to allow updates as well as disabling the shared_info pages.
We're going to need that to implement SHUTDOWN_soft_reset for kexec.

> > 
> > If that was allowed, wouldn't it have been a much simpler fix for
> > CVE-2019-3016? What am I missing?
> 
> Agreed.
> 
> Perhaps, Paolo can chime in with why KVM never uses pinned page
> and always prefers to do cached mappings instead?
> 
> > 
> > Should I rework these to use kvm_write_guest_cached()?
> 
> kvm_vcpu_map() would be better. The event channel logic does RMW operations
> on shared_info->vcpu_info.

I've ported the shared_info/vcpu_info parts and made a test case, and
was going back through to make it use kvm_write_guest_cached(). But I
should probably plug on through the evtchn bits before I do that.

I also don't see much locking on the cache, and can't convince myself
that accessing the shared_info page from multiple CPUs with
kvm_write_guest_cached() or kvm_map_gfn() is sane unless they each have
their own cache.

What I have so far is at

https://git.infradead.org/users/dwmw2/linux.git/shortlog/refs/heads/xenpv

I'll do the event channel support tomorrow and hook it up to my actual
VMM to give it some more serious testing.


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